1. Field of the Invention
The present invention relates to an optical measurement technology and particularly to an optical measurement method and apparatus for acquiring biological information based on optical signals received from a tissue.
2. Background Art
Measurement signals (to be hereafter referred to as an optical signal) obtained by optically acquiring internal information about a living tissue (such as in JP Patent Publication (JP Patent Publication (Kokai) No. 9-135825 A (1997)) contain noise signals due to several factors, in addition to the target response signal. To reduce the influence of such noise signals, averaging or other techniques using a bandpass filter, for example, are used.
The averaging method for reducing noise signal is based on the premise that the brain activities show uniform responses to the same tasks, for example. The method employing a bandpass filter for reducing noise signal is based on the premise that a response signal and a noise signal exist in different frequency bands. These are processes for dealing with multiple noise signals. Processes attuned to noise signals due to specific causes have also been considered.
For example, in the case of a noise signal due to the influence of pulsation (pulsation noise signal), the frequency can be easily identified, so that the noise signal can be reduced using a bandcut filter. In another example, the pulsation noise signal contained in an optically measured signal is reduced on the basis of a pulsation signal measured at a certain location, such as at the ear (Patent Document 2). In yet another example, a pulsation noise signal is extracted from the optically measured signal and its influence is reduced (Non-patent Document 1). However, the actual brain activities do not necessarily produce uniform responses to the same tasks, and the signal processing scheme using averaging does not always produce successful results.
Even if the signal processing based on averaging is effective, it takes much time for acquiring signals to be summed. Further, some noise signals in the optical signal contain irregular components with a frequency band of approximately 0.1 Hz. These components are a kind of fluctuation signal that is inherently possessed by living bodies and are referred to as a low-frequency fluctuation signal or the Mayer wave.
Regarding the Mayer wave, a number of studies have so far been conducted, and the involvement of fluctuations in blood pressure and heart beat or heart rate has been indicated. However, not much light has been shed on the details of its occurrence mechanism. In this connection, it is noted that in the case of Patent Document 1, noise is eliminated by subtraction using a pulsation signal, and therefore the Mayer wave noise component, which cannot be readily obtained from the pulsation signal, cannot be eliminated by subtraction of the pulsation signal.
Patent Document 1: JP Patent Publication (Kokai) No. 9-135825 A (1997)
Patent Document 2: JP Patent Publication (Kokai) No. 2004-173751 A
Patent Document 3: JP Patent Publication (Kokai) No. 7-308295 A (1995)
Patent Document 4: JP Patent Publication (Kokai) No. 2002-172094 A
Non-patent Document 1: Maria Angela Franceschini et al., NeuroImage 21 (2004) 372-386